This invention relates to acoustical flowmeter systems and is particularly directed to an improvement in the acoustical flowmeters of the type described and claimed in the U.S. Pat. No. 4,003,252 entitled "Acoustical Wave Flowmeter" by E. J. DeWath which issued Jan. 18, 1977 and the flowmeter system of the type described and claimed in the U.S. Pat. No. 4,164,865 entitled "Acoustical Wave Flowmeter" by L. G. Hall and R. S. Loveland which issued Aug. 21, 1979.
The invention of DeWath was directed to a flow meter having an unobstructed tubular wall thereby eliminating all impediments to the flow path of the fluid and eliminating all cavities in which debris might collect. The advantages of such a configuration is fully set forth in the DeWath patent. To measure flow of a selected fluid in the DeWath flowmeter, however, required a calibration for that particular fluid and required a recalibration if the flow of a different fluid was to be measured since the flowmeter was not responsive to changes in fluid species or densities.
The Hall and Loveland invention improved the DeWath flowmeter by providing a flowmeter that measured flow accurately regardless of changes in fluid composition or temperature and by providing a flowmeter with a means for determining a change in velocity of sound of the fluid being measured.
In order to accomplish this, the Hall and Loveland acoustical wave flowmeter system had two spaced apart crystal transducers in the wall of the flowmeter conduit (sometimes called a cavity) to produce ultrasonic acoustic compressions at selected frequencies in the fluid within the cavity. The transducers were alternately switched into a transmit and a receive mode to generate upstream and downstream transmitted and received signals with an automatic means to adjust the transmitted frequencies to compensate for changes in velocity of the acoustic compressions in the fluid caused by changes in fluid composition and temperature. The electronic circuitry involved in the Hall and Loveland flowmeter system include means for measuring and storing signals representing the phase difference between the transmitting transducer signal producing the acoustic compressions and the signal produced by the receiving transducer during each of two successive transmit/receive cycles. Circuit means were provided to determine the difference between the signals representing the two successive phase differences wherein the sign of the difference corresponds to the direction of the fluid flow and the magnitude of the difference corresponds to the rate of fluid flow through the flowmeter. Circuit means were also provided to add the two successive phase difference signals together to obtain a signal proportional to the velocity of sound in the fluid moving through the flowmeter. This latter signal indicated the change in composition of the fluid flowing through the meter.
The Hall and Loveland flowmeter system had a phase lock loop in the receiver/transmitter system which included, among other circuit components, a phase detector, voltage controlled oscillator (VCO) and a loop filter. This loop filter was a passive filter of the RC type for filtering the error voltage signal applied to the VCO which would respond by changing the transmitted frequency of the transducer. The problem encountered with this system is that, once calibrated to operate at a certain fluid density, a change in fluid density, for example, would cause the VCO to operate at a different frequency which means that the phase detector has a constant phase error to create the voltage to drive the VCO to a new frequency and thus the range of the phase detector for measuring the magnitude of flow was thus limited. For example, if the offset, or voltage applied to the VCO, were to change one volt, this would mean that the output of the phase detector would be required to work at a greater phase error difference from that for which the system was calibrated. Thus, with less phase error to work with, the flow measurement range is decreased, making the system more sensitive to changes in fluid flow or density which could cause the entire system to go out of range or into an out-of-lock mode.
This invention improves the prior system by requiring only a very small phase error to be detected by the phase detector in order to change the error signal applied to the VCO by a large amount thus improving the loop gain of the system. Accordingly, it is a primary object of this invention is to improve the loop gain of a phase lock loop circuit in a flow meter system.